Adam Clarke on Daniel 1:8

But Daniel - would not defile himself - I have spoken of this resolution in the introduction. The chief reasons why Daniel would not eat meat from the royal table were probably these three: -

1. Because they ate unclean beasts, which were forbidden by the Jewish law.

2. Because they ate, as did the heathens in general, beasts which had been strangled, or not properly blooded.

3. Because the animals that were eaten were first offered as victims to their gods. It is on this account that Athenaeus calls the beasts which here served up at the tables of the Persian kings, ἱερια, victims, lib. 4 c. 10, p. 145.